1. Field of the Invention
The present invention relates to global positioning satellite (GPS) signal reception, and more particularly to a method and an apparatus for detecting a data-bit boundary to discriminate whether a satellite signal is received.
2. Description of the Related Art
A satellite signal transmitted from a satellite to a GPS receiver includes a satellite pulse train, which is the product of a satellite data stream and a Pseudo-Noise (PN) Code. GPS receivers calculate the range from the receiver to a satellite. Once the range to a satellite is known, the receiver knows it lies somewhere on a sphere of radius equal to this range. If the range to a second satellite is found, a second sphere can be drawn around that satellite (diagram). The receiver now knows that it lies somewhere on the circle where the two spheres intersect. With a third satellite, the location can be reduced to two points. To calculate the range from the receiver to the satellite, two things are needed: time and speed. The satellites send out a continuous radio signal, picked up by the receiver which multiplies the speed of the signal, (the speed of light) by the time it took the signal to travel from the satellite to the receiver. All GPS satellites have several atomic clocks. The signal that is sent out includes a random sequence called pseudo-random (PRN) or Pseudo-Noise (PN) code. This pseudo-random sequence is repeated continuously. All GPS receivers know this sequence and repeat it internally (as a replica PN code). The receiver picks up the satellite's transmission and compares the incoming signal to its own internal signal. By comparing how much the satellite signal is lagging, the travel time becomes known. Along with the pseudo-random Pseudo-Noise (PN) code, the satellite also transmits a “navigation message” data stream containing its exact orbital characteristics. The receiver on the ground takes this information and uses it to plot the satellite's location. The unique navigational data signal each GPS satellite transmits is a centered on two L-band frequencies of the electromagnetic spectrum. The navigation message contains the satellite orbit information, satellite clock parameters, and pertinent general system information necessary for real-time navigation to be performed. The navigation message generally must be decoded before the receiver starts the tracking cycle for real-time positioning (note that the message only changes once an hour).
The satellite data stream includes data-bits and thus a data-bit boundary (boundaries). Since there is initially uncertainty about the bit boundaries in the transmitted data bits in a GPS navigation message, this uncertainty must be removed in order to be able to detect navigation data bits. If receiver position and time is roughly known, these can be used for bit synchronization, otherwise bit boundaries have to be detected.
Traditionally GPS receivers have been designed with separate acquisition and tracking modes. To compute a position, the device must first acquire the satellite signals, and to do this it generally must search over all possible frequency and code-delay bins. The GPS receiver detects the signal by correlating (coherent integration). That is, multiplying the received signal with a locally generated replica of the PN code used in the satellite, and then integrating (or low-pass filtering) the product to obtain a peak correlation signal. The peak of this signal vanishes when the locally generated code-delay is wrong, or when the frequency is wrong. The GPS receiver correlates the incoming (received) signal with versions of the locally generated (replica) PN code shifted by different phase amounts. Thus, to acquire the signal, a GPS receiver must search the entire space of possible frequency offsets and code delays. The search is conducted over ranges of frequency and code-delay, which we call bins.
The L1 carrier is modulated with a 10.23 MHz precise (P-code) ranging signal and a 1.023 MHz clear acquisition (or coarse/acquisition, C/A code) ranging signal. These are pseudo random noise (PRN) (PN) codes in phase quadrature. The L2 signal is modulated with the P-code only. Both the L1 and L2 signals are also continuously modulated with a data stream at 50 bits per second. Access to GPS by civilian users is provided through the C/A coded signals. The C/A code is modulated by a PRN Gold code of 1023 chips, at a chipping rate of 1.023 MHz, resulting in a null-to-null bandwidth of 2.046 MHz and a repetition rate of 1 millisecond. Each satellite has its own unique C/A code that provides satellite identification for acquisition and tracking by the user.
The navigation message consists of a 50 bit per second data stream containing information enabling the receiver to perform the computations required for successful navigation and is repeated every 30 seconds. Because the data rate is 50 bits/second, each bit lasts for 20 milliseconds. If these bits are not known, then a coherent average accumulated before the bit boundary may have a different sign then the average accumulated after the bit boundary. In a conventional satellite-signal reception discriminating apparatus, the position of a satellite must be detected by separately receiving information corresponding to the data-bit boundaries because the data-bit boundary cannot be detected in a correlation integration process. Also, in another conventional satellite-signal reception discriminating apparatus, in order to detect the position of the satellite, a correlation integration by parts of the satellite pulse trains and a replica PN code is performed and the square of the resulting integration values is computed. This method is known as a noncoherent integration process. Then, the conventional satellite-signal reception discriminating apparatus detects the position of the satellite using the squared value. However, in case where the noncoherent integration process is performed, the noise also contained in the satellite signal becomes larger as the satellite signal becomes larger. Therefore, the efficiency thereof is decreased. The conventional satellite-signal reception discriminating apparatus performs a coherent integration process to eliminate the noise. However, in the conventional satellite-signal reception discriminating apparatus, where the coherent integration process is employed, the process of the correlation integral by parts is performed without the square of the resulting integration values. Subsequently, the conventional satellite-signal reception discriminating apparatus cannot detect the data-bit boundary in the coherent process. As a result, the conventional satellite-signal reception discriminating apparatus determines that the satellite signal is not being received even though the satellite signal is being received from a desired satellite. Therefore, there is a need for an apparatus that can detect the data-bit boundary and discriminate the satellite signal corresponding to the data-bit boundary.